If we take any selection of four digits, ex. (4,9,4,8), we can always arrange the four is ascending order: (4,4,8,9). Thus, all we need to finish is to determine how many possible four-selections are there. and there are possible pin numbers.

If we take any selection of four digits, ex. (4,9,4,8), we can always arrange the four is ascending order: (4,4,8,9). Thus, all we need to finish is to determine how many possible four-selections are there. and there are possible pin numbers.

I understand that there are 10^4 possible pin numbers but i do not understand what means.

If we have Ndifference types of items from which we are to select K items, we may pick more than one of any type.
For example you are to pick four pieces of fruit from apples, oranges, or bananas.
You may pick two apples and two bananas.